1. Field of the Invention
The present invention relates to a method of fabricating Cu.sub..alpha. (In.sub.x Ga.sub.1-x).sub..beta. (Se.sub.y S.sub.1-y).sub..gamma. film for solar cells.
2. Description of the Prior Art
Solar cells in recent years have achieved a high maximum conversion efficiency of 17.7% by using films consisting of Cu.sub..alpha. (In.sub.x Ga.sub.1-x).sub..beta. (Se.sub.y S.sub.1-y).sub..gamma. (hereinafter also referred to as "CIGS"). However, when such films are grown under conditions in which there is an excessive supply of group III elements In and Ga, it is possible to fabricate single phase CIGS, but it has a high defect density and high resistance that degrade the properties of the resultant solar cell. On the other hand, while using an excess supply of Cu, a group I element, does provide large, good-quality crystals, it also results in the surface and boundary formation of the low resistance Cu--Se metal phase that makes devices prone to short circuit.
In order to grow high-quality CIGS films for solar cells, a complicated method is currently used, comprising first using an excess supply of Cu to form large-grain, high-quality CIGS, and followed by a step of using excess Ga and In to thereby remove a Cu--Se phase on the surface. Moreover, since a high temperature of around 550.degree. C. is used to ensure the adequate reaction of each element, the method can only be used with substrates able to withstand such temperatures.
An object of the present invention is to provide a method of fabricating CIGS film for solar cells in which the generation of point defects (divacancies) and twin-crystal stacking faults is suppressed to thereby fabricate high-quality film.
Another object of the invention is to simplify the formation process by providing a method of fabricating high-quality CIGS films for solar cells in which the film is formed at a low temperature.